INTRODUCTION
Synthetic dyes are potential water pollutants that are widely used by the textile industry; they cause problems because of their toxicity and their coloring (Han et al. 2015; Singh et al. 2022). Adsorption is an effective way to remove dyes from wastewaters, and activated carbon is widely used for this purpose. However, the activation process increases the cost and energy consumption of the process. Chemical activation is widely used to produce activated carbon from biomass. However, using chemicals for the activation increases the cost of the process and causes hazardous effects to the environment (Choi et al. 2019; Vigneshwaran et al. 2021). Therefore, using an adsorbent that is abundant in nature, or a waste or a by-product of a process becomes important. Biochar is carbon-rich material that is obtained via thermochemical conversion of biomass. Despite having low adsorption capacities, biochar is a potential alternative to activated carbon in water treatment due to its low cost and not using chemicals for the activation (Zazycki et al. 2018; Choi et al. 2019). Agricultural byproducts that have low economic value and biochar produced from those materials are good alternative adsorbents for the removal of dyes from wastewater.
Black cumin is used in the treatment of cancer, diabetes, asthma, and kidney diseases due to the different active pharmaceutical components in its content. It is grown in Southwest Asia and Eastern Mediterranean countries (Zent 2019; Yimer et al.2019; Can 2020). In Turkey, black cumin is grown in provinces such as Mersin, Gaziantep, Isparta, Burdur, Uşak, and Bursa (Koşar and Özel 2018; Can 2020). While black cumin seed production in Turkey was 161 tons in 2012 (2299 decares), it increased to 3603 tons in 2019 (37085 decares). When considering the foreign trade data of black cumin, it can be seen that 2647.5 tons of black cumin seed ($2.53 million) was imported in the same year, equal to the export of 592.4 tons ($1.23 million) of black seed in 2019 (Can 2020). Black cumin is used as a health supplement and condiment, as well as spice in the food industry, and the pulp is used as a filler in the production of environmentally friendly polymer composites (Yimer et al.2019; Sismanoglu et al. 2022). It is used as feed in animal nutrition in order to prevent environmental pollution and create added value from the pulp part that is released during the production of black seed oil (Zent 2019).
A natural agricultural product, Lolium perenne seed, was used for the removal of safranine T, and the adsorption capacity qmax was 323 mg/g at 25 °C and pH 7 according to the Langmuir isotherm model (Karadeniz et al. 2023). In another study, Cotinus coggygrialeaves were used as an adsorbent for the removal of safranine T. The maximum adsorption capacity was 2000 mg/g at 55 °C and pH=7 (Ugraskan et al. 2022). Temiz and co-workers produced microbeads composites with the combination of Capsella bursa-pastoris and chitosan for the removal of methylene blue, and the maximum adsorption capacity was reported as 222 mg/g at 25 °C. (Temiz et al. 2022). Valeriana officinalisroots were used as an adsorbent for the adsorption of anionic Congo red and cationic crystal violet dyes. At 25 °C, the Langmuir qmaxvalues were 166.7 and 476.2 mg/g for Congo red and Crystal violet, respectively (Akdemir et al. 2022). In another study, Rumex acetosella was used as an adsorbent for adsorption of crystal violet, and the maximum adsorption capacity was reported as 434.8 mg/g at 25 °C and pH=7 to 8 (Erdogan et al. 2022). Judas tree (Cercis siliquastrum) seeds were used as an adsorbent in the adsorption of Basic blue 9 and Basic green 4 dyes. The qmaxvalues for the Langmuir isotherm were reported as 500 and 244 mg/g at 25 °C for BB and BG, respectively (Isik et al. 2022).
Compared with other studies with basic yellow 28 (BY28), the adsorbent (GAC-HB) was added to 200 mL of basic yellow 28 dye solution on coconut shell charcoal granular activated carbon. Different studies were conducted on sorbent dose, initial dye concentration, solution pH 9, GAC-HB dose 0.8 g/L, and contact for 2 h. The optimum conditions for BY28 were determined. The Langmuir qmax value was 769 mg/g (Dao et al. 2013). Activated carbon produced from Persea americana nut (PAN) was used with V= 50 mL BY 28 (100 mg/L) in at ambient temperature and Langmuir maximum uptake (qmax) at 0.2 g/L dosage was 325 mg/g (Regti et al.2017). The adsorbent is related to the adsorption of BY28 to low-cost natural red clay (NRC). For adsorption studies, contact time, initial concentration, pH, adsorbent mass, and temperature were studied as 1 to 180 min, 50 to 300 mg/L, 3-10, 0.5 to 10 g/L, and 25 to 40 °C, respectively. Langmuir maximum monolayer adsorption capacity qmax was 370 mg/g. The adsorption of basic yellow 28 dye on adsorbent Ca-bentonite was investigated at pH 7.0 for temperatures of 20 and 40 °C. The Langmuir maximum monolayer adsorption capacity (qmax) of Ca-bentonite was determined as 94.3 mg/g at 20 °C and 99.0 mg/g at 40 °C (Kalpaklı et al. 2014).
In another study, adsorption of methyl violet (MV) onto biochars from crop residues showed the following results: peanut straw charcoal > soybean charcoal > rice husk charcoal. They were rinsed with 0.200 g biochar in a water bath at a constant temperature of 25 ± 1 °C for 2 h and after 22 h it was adjusted at pH 8 to 10 and centrifuged. The Langmuir maximum uptake (qmax) was 256, 178, and 123 mg/g, respectively for peanut charcoal > soybean charcoal > rice husk charcoal (Xu et al. 2011). Adsorbent Casuarina equisetifolia needle (CEN) was used to extract methyl violet 2B (MV) from aqueous solutions. Optimal conditions were at room temperature with a contact time of 2 h and no pH adjustment was required. From the experimental data, the maximum adsorption capacity was found as 165 mg/g for the Langmuir model (Dahri et al. 2013). Modified Ceiba pentandra sawdust has been investigated as an adsorbent for the removal of methyl violet dye from aqueous solutions. The effective pH for methyl violet adsorption was 7. Equilibrium was reached in 30 min. The Langmuir maximum capacity (qmax) was 16 mg/g (Astuti and Fatin 2018). Pine bark was activated with HNO3 as an adsorbent and used as activated carbon (AC) to remove textile dye (methyl violet MV) from aqueous solutions. The variables were the pH of the solution (3 to 11), activated carbon (from pine bark) amount (0.01 to 0.13 gm), MV dye concentration (5 to 50 mgL-1), and solution temperature (10 to 50 °C). The maximum adsorption capacity of the dye MV at AC was 95.13 mg/g (Bader et al. 2019). The adsorption of methyl violet and malachite green 2B by phthalate-functional sugarcane pulp (SPA) was investigated. SPA (5, 10, 15, 20, 25, 30, 35) mg was added to 20 mL of 25 mg L-1 methyl violet 2B solution, shaken again at 180 rpm for 180 min. The qmax of MV 2B was 60.5 mg g-1 (Ariani et al. 2018).
In this study, black cumin seed pulp, which has a low economical value, and biochar, which was produced via pyrolysis of black cumin seed pulp, were used as low-cost adsorbents for the removal of methyl violet 2B and basic yellow 28. The characterization of the adsorbent was performed before and after adsorption by FTIR/ATR, TGA, and SEM. The removal of methylene violet 2B (MV) and basic yellow 28 (BY28) were tested, and different kinetic and adsorption isotherms models were applied.
EXPERIMENTAL
Feedstock and Chemicals
Black cumin (Nigella sativa) seed pulp was obtained at no cost from Bağdat Spices Co., Ankara, Turkey, and it was used as received. Methyl violet 2B (CAS number: 8004-87-3, molecular weight: 393.95 g/mol) and basic yellow 28 (CAS number 54060-92-3, molecular weight: 433.52 g/mol) were used in the adsorption experiments. Basic yellow 28, which is used in textiles with azo group, ionizes in its aqueous solutions and forms positively charged ions and is therefore a cationic dye (Slimani et al.2014; Benkaddour et al.2020). Methyl violet 2B, known as triphenylmethane dyes because it has 3 aryl groups attached to a nitrogen atom interacting with one or two methyl groups, is a basic dye due to the positive charge on the amino group, and its aqueous solutions are purple while the powder is dark green (Bonetto et al.2015; Dahri et al.2015). The chemical formulas of active pharmaceutical ingredients in black cumin thymol and the molecular structures of BY28 and MV 2B are shown in Figs. 1 and 2, respectively.
Enlarge this image.Fig. 1. Chemical formulas of active pharmaceutical ingredients in black cumin thymol (a), thymoquinone (b), limonene (c), carvacrol (d), t-anethole benzene (e), 4-terpineol (f), α-pinene (g), longifolene (h), p-simen (i) (Sismanoglu et al. 2022)
Production of Biochar
The biochar was produced viapyrolysis of black cumin seed pulp at 500 °C with a heating rate of 10 °C/min. The pyrolysis experiments were carried out in a 250 mL stainless steel fixed bed reactor in which continuous flow of nitrogen gas was used to provide an inert atmosphere. In the pyrolysis experiment, 10 g of black cumin seeds (dry basis) was placed in the reactor. Before heating the reactor to 500 °C with a heating rate of 10 °C/min, the air inside the reactor was purged with nitrogen flow for 15 min. When the temperature reached to 500 °C, the reactor was held at that temperature for 1 h and then the heating was stopped. After cooling of the reactor to room temperature, the reactor was opened, and biochar was taken from inside the reactor. No further treatment was applied to the biochar before the adsorption experiments.
Characterization of Adsorbates and Adsorbents
The changes of adsorbates and adsorbents surfaces before and after adsorption were characterized by FTIR and SEM. In addition, thermal behavior before and after adsorption was analyzed by TGA. FTIR/ATR analysis was performed on the Alpha Bruker FTIR spectrometer, in the range of 4000 to 500 cm-1, in ATR mode. SEM micrographs were taken by Carl Zeiss Ultra Plus Gemini field emission scanning electron microscope (FESEM) (Oberkochen, Germany) at a X20000 magnification. Before the experiment, the powders were covered with gold sputtering for 15 min. Thermal analysis of adsorbents and adsorbates was carried out using the Hitachi STA 7300 Thermogravimetric Analyzer in the temperature range of 25 to 600 °C in an inert nitrogen atmosphere at a heating rate of 10 ˚C/min and a nitrogen gas flow of 20 mL/min.
Batch Adsorption Experiments
Solutions of BY28 150, 100, 75, 50, 25 ppm and MV 200, 150, 100, 50, 25 ppm concentrations were made in a shaking water bath for 10, 25, 35 °C at certain intervals for a total of 60 min.
Enlarge this image.Fig. 3. Graphs of qe (mg/g) vs. time of BY28-C at 10 °C (a), 25 °C (b), 35 °C (c); BY28-CC at 10 °C (d), 25 °C (e), 35 °C (f)
Enlarge this image.Fig. 4. Graphs of qe (mg/g) vs. time of MV-C at 10 °C (a), 25 °C (b), 35 °C (c); MV-CC at 10 °C (d), 25 °C (e), 35 °C (f)
The equilibrium time of adsorption was chosen as 30 min for BY28 (Fig. 3) and 60 min for MV (Fig. 4) from the qe vs. t plot. A total of 10 mL of dye concentrate, 0.02 g of black cumin, and black cumin char were used. The dose rate was 2 g/L. The pH for BY28 was 5.5 to 6, while the pH of MV was 5 to 6. The amount of dye collected on unit absorbent, qe (mg/g), calculated as follows,
(1)
where ce is the dye concentration remaining in solution after adsorption (mg/L), c0 is the initial concentration of dye (mg/L), V is the total dye volume (L), and m is the adsorbent amount (g).
For adsorption isotherms, Freundlich, Langmuir, Temkin, Dubinin and Radushkevich (D-R), and Flory-Huggins (F-H) isotherms were used. In thermodynamic calculations, the standard Gibb’s energy was calculated with the following equation (Malkoc and Nuhoglu 2007),
(2)
(3)
where Kads(L/g) is the standard thermodynamic equilibrium constant. Using this equilibrium constant, with the following equation,
(4)
where lnKads versus 1/T enthalpy from the slope of the plot provided the heat of reaction (ΔHo kj/mol), and the entropy (ΔSo j/molK) was obtained from the intercept with the vertical axis (Malkoc and Nuhoglu 2007; Rahchamani et al. 2011).
Isotherms
Freundlich
According to this isotherm, the adsorption areas on the surface of an adsorbent are heterogeneous; it is composed of different types of adsorption sites. The Freundlich isotherm equation in linear form is as follows,
(5)
where KFis the adsorption capacity (L/mg), and n is the adsorption density (Hakan Duran et al. 2019). When n<1, adsorption is favorable (Reed and Cline 1994).
Langmuir
This isotherm assumes that there are trapping points on the adsorbent surface. Assuming that each holding point will adsorb a dye molecule, the layer formed has a thickness of one molecule. In equilibrium, it reaches the maximum adsorption capacity and the surface is covered with a monolayer (Duran et al. 2019), as follows,
(6)
where qmax is the maximum adsorbing capacity of the adsorbent (mg/g).
Temkin
According to this isotherm, the heat of adsorption decreases linearly with the coating of the surface due to the interactions between the adsorbed dye molecules, and the participation of this heat during the adsorption will be either endothermic or exothermic (Malkoc and Nuhoglu 2007).
(7) (8) (9)
where bT is the Temkin constant (kj/molK), B is the adsorption potential (energy) related (kJ/mol), KT is the Temkin isotherm constant (L/mg), R is the gas constant (J/molK), and θ is the ratio of the components adsorbed on the surface of the adsorbent.
The equilibrium amount (qe mg/g) of adsorbed dyes on the surface of the adsorbent is divided by the monolayer adsorption capacity (qmax mg/g) from Langmuir curves. ΔQ is the adsorption energy (kJ/mol). Assuming that the process is carried out under a closed system, the heat value ΔQ is almost the same as ΔH. As a thermodynamic formula, ΔQT = – ΔH. The ΔH value can be defined as follows: ΔH > 0 kJ/mol are endothermic processes, and ΔH < 0 kJ/mol are exothermic processes (Pursell et al. 2011; Kireç et al. 2021).
Dubinin and Radushkevich (D-R)
According to this isotherm, a retained layer exhibits multi-layer character, and the following expression is used to calculate the adsorption energy of the adsorption system (Savran et al. 2017),
(10)
where KD-R is the adsorption energy corresponding constan
t (mol2kJ2), and ε is the Pollanyi constant, calculated with , and KD-R is the adsorption energy calculated using this formula by using the relevant constant, as shown below. (11)
It has been stated that if this energy is within the range 8 kJ/mol < E < 16 kJ/mol, the adsorption mechanism is ionic exchange, if E < 8 kJ/mol, the adsorption mechanism is physical adsorption, and if E > 16 kJ/mol, the adsorption mechanism is diffusion (Savran et al. 2017).
Flory-Huggins (F-H)
While deriving the degree of coating of the surface by the dye on the adsorbent, this isotherm informs about the feasibility of the adsorption mechanism, that is, whether it is spontaneous or not (Saadi et al. 2015),
(12)
where θ is the degree of surface coverage, KF-H is the isotherm constant, and n is the number of ions at sorption sites. Equation 13 defines the degree of surface coverage.
(13)
The Gibb’s energy is calculated using the KF-H constant.
(14)
If ΔG (kj/mol) < 0, the adsorption mechanism will be spontaneous; if ΔG (kj/mol) > 0, the adsorption mechanism will not be spontaneous (Saadi et al. 2015).
Kinetics
Pseudo First Order Kinetic Model
The pseudo-first order kinetic model was given by Lagergren (1898) for the adsorption of solid/liquid systems and its formula is as follows,
(15)
where k1 is the first order adsorption rate constant (min-1), qt is the amount of substance adsorbed at time t (mgg-1), and qe is the amount of substance adsorbed (mgg-1) at equilibrium. The equation with integral and boundary conditions applied is given by:
The basis of the second order kinetic model is based on the adsorption capacity of the solid phase (Hubbe et al. 2019), as follows, (17)
where k2 is the second order adsorption constant (g mg-1 min-1). At t=0, qt=0. At t=t, qt= qtboundary conditions are applied and equality integral is taken, as follows.
(18)
According to McKay and Ho the linearized form of the equation (Ho et al. 1999) can be expressed as,
(19) In addition, when looking at the groups present in the black cumin structure (Fig. 1); C-O tension is seen at 1064 cm-1, and the aromatic extra-ring C-H bend and alkene C-H bend functional group wavelength is 668 cm-1(Nandiyanto et al. 2019). In the BY28 dyestuff structure, it was observed that the C-S tension of CH3OSO3 and the C-H and C-O groups of black cumin increased the peak intensities at 1064 cm-1 and 668 cm-1 more than the MV dye. From this, it was thought that BY28 was more adsorbed in black cumin.
SEM Analysis Results
Black cumin has a slotted and mounded structure, while char has a scaly and channeled structure. After the adsorption of BY28 on black cumin (Fig. 6a), the BY28 dye molecules settled into the slits and covered the surface, and the structure became smoother.
After the adsorption of MV on black cumin (Fig. 6c), a wavy structure was observed in which the dye molecules completely covered the surface, and the slits were closed. After the adsorption of char with BY28 (Fig. 6b), the surface porosity of the char increased, and the dyestuff was stuck on the surface. After adsorption with MV, channel depths were reduced by MV molecules, and its scaly structure was covered by MV (Fig. 6d). A flatter structure was formed in the adsorption of char with MV (Fig. 6d).
TGA Results
TGA curves for black cumin seeds and biochars before and after adsorption are shown in Fig. 7. The results were similar for black cumin seeds before and after adsorption. The evaporation of moisture was observed with the weight loss up to 135 °C. The further weight loss until 600 °C includes the thermal decomposition of hemicellulose, cellulose, and lignin. The thermal degradation of biochars was roughly similar. The evaporation of absorbed water in biochars was observed with the weight loss up to 200 °C. The weight loss at between 200 and 450 °C can be attributed to the degradation of cellulose, hemicellulose, and other volatile components. The weight loss above that temperature reflects the degradation of residual lignin (Choudhary et al. 2020; Sismanoglu et al. 2022; Deng et al. 2022). When the thermal behavior graphs of the adsorption of dyestuffs with pure black cumin and pure black cumin in Figs. 7a and 7c are examined, the stages seen in the graphs are due to the impurities in the black cumin structure. In Figs. 7b and 7d, it can be seen that the impurities in black cumin decreased with pyrolysis and the temperature resistance increased in the thermal behavior graphs of the adsorption of char and char and with dyestuffs. It was observed that the weight loss (%) in the TGA curve of the adsorption experiments performed with black cumin was higher and the thermal resistance decreased accordingly. When the experiments with char were examined, it was seen that the thermal resistance values before and after adsorption were close to each other and the thermal resistance was slightly higher for both dyes after adsorption at high temperatures. These results are also consistent with the qmax values in Tables 1 and 2.
Shapes of the Isotherms
The ce graphs (Figs. 8 and 9) drawn against qe from the adsorption isotherm curve of both dyestuffs on the adsorbents conformed to the Giles isotherm L type. Looking at the graphs, it becomes difficult for the dye molecules to find empty space as the empty spaces on the first curve (layer) part of the adsorbent (black seed and char) are filled rapidly. Therefore, the second curve (layer) is formed. This means that a new monolayer is formed on top of the first layer (Giles et al. 1960). Thus, the dye molecules, rather than the solvent molecules, are more aggressively directed towards the adsorbent.
Enlarge this image.Fig. 8. Graphs of qe (mg/g) vs. ce of BY28-C at 10, 25, and 35 °C (a); BY28-CC at 10, 25, and 35 °C (b)
Enlarge this image.Fig. 9. Graphs of qe (mg/g) vs. ce of MV-C at 10, 25, and 35 °C (a); MV-CC at 10, 25, and 35 °C (b)
Adsorption Isotherms
KF in the Freundlich isotherm represents the ability of adsorbates to adhere to adsorbents. KF is the adsorption capacity in the Freundlich adsorption isotherm, and it can be meaningfully matched with qmax in the Langmuir isotherm. KF has no numerical range, such that only a comparison is made of the values found as a result of the calculation. The adhesion ability of MV on black cumin was found to be higher than BY28 for all 3 temperatures (Tables 1 and 2) and KF values for both dyes decreased with temperature. For black cumin char, the KF was high for MV at 3 temperatures, whereas the KF values for BY28 and MV changed irregularly with temperature. The adsorption intensity of both dyes on adsorbents was n<1, indicating that adsorption was favored. Electronegativity is the force of attraction of electrons used in bond making by the atoms forming the bond. Atoms whose external energy levels are almost completely filled, such as chlorine, are strongly electronegative and readily gain electrons. The fact that the molar mass of MV is lower than that of BY28 may cause more adhesion on the adsorbent’s surface. Since the electronegativity of the Cl– anion in the structure of the MV is high, it can be said that the adsorbent attracts the electrons on the surface more easily and the retention on the surface is therefore increased. While the compliance of BY28 to the Freundlich isotherm (R2) of adsorption on both adsorbent at three different temperatures was between 97% and 98% for black cumin, it was between 87% and 91% for black cumin char. While the MV was between 92% and 99% for black cumin, it was between 91% and 98% for black cumin-char.
As shown in Tables 1 and 2, the qmaxvalue of BY28-C for all three temperatures was quite high compared to MV-C. The qmax values of BY28-C and MV-C at 25 °C were 212.8 mg/g and 164 mg/g, respectively. When compared with other temperature values, it can be seen that the maximum adsorption capacity (qmax) of black cumin in the highest monolayer was 25 °C. The qmax values of black cumin char of BY28 and MV (BY28-CC, MV-CC) at 25 °C were 625 mg/g and 909 mg/g, respectively. The qmax value of the char at 25 °C was higher than the values at other temperatures. According to Tables 1 and 2, the qmaxof the BY28-C at 25 °C was higher when the MV-C was compared with the BY28-C. Likewise, when MV-CC and BY28-CC were compared according to Tables 1 and 2, the qmax value of MV-CC at 25 °C was higher. As a result, while the qmax(mg/g) data of the adsorption capacity in the Langmuir model preferred BY28 in the raw form of black cumin; it showed that the one in the char state preferred MV. While the compatibility of BY28 to the Langmuir isotherm (R2) of adsorption on both adsorbent at three different temperatures was between 98% and 99% for black cumin, it was between 88% and 96% for black cumin-char. The suitability of MV to Langmuir isotherm (R2) was between 96% and 99.8% for black cumin; it was between 97% and 99% for black cumin-char.
When the Temkin isotherm is applied for BY28-C, MV-C, BY28-CC, and MV-CC, the adsorption type of BY28 and MV on adsorbents is physical adsorption, since B, that is, the adsorption potential (energy) in the first equation of Temkin, is lower than 8 kJ/mol (Tables 1 and 2). The ΔQ adsorption energy is almost the opposite sign of the ΔH adsorption heat value result. Using the second equation of the Temkin, it was calculated that the adsorption heat of BY28 and MV dyestuffs on both adsorbent is exothermic. For black cumin and char, in which basic yellow 28 was used as dyestuff, the equivalence of the Temkin isotherm equation 7 (R2) was between 92% and 93%, while it was between 93% and 99% for black cumin and char, respectively. When BY28 was used, the equivalence of the Temkin isotherm equation 9 (R2) was between 93% and 95% for black cumin, while it was between 97% and 98% for black cumin char. The equivalence of MV adsorption on both adsorbent at three different temperatures to the Temkin isotherm equation 7 (R2) is between 88% and 96% for black cumin, while it is between 86% and 94% for black cumin char. The equivalence conformity to the Temkin isotherm equation 9 (R2) is between 91% and 96% for black cumin, and 86% to 94% for black cumin char.
Dubinin and Radushkevich (D-R) adsorption isotherm results, as shown in Tables 1 and 2 qmax, that is, the maximum saturation capacity for BY28-C is 41 to 42 mg/g; 42 to 53 mg/g for BY28-CC; 51 to 56 mg/g for MV-C; For MV-CC, it is 57 to 67 mg/g. Since the calculated adsorption mechanism energy (E) of both dyes on the adsorbents is lower than 8 kJ/mol, it is seen that physical adsorption takes place. This value is compatible with the B value in Temkin’s first equation. The conformity of BY28 to the D-R isotherm of adsorption at three different temperatures on both adsorbent (R2) is between 85% and 86% for black cumin, while it is between 93% and 97% for black cumin-char. The suitability of adsorption of MV on both adsorbent at three different temperatures to the D-R isotherm (R2) is between 71% and 81% for black cumin, while it is between 75% and 82% for black cumin-char.
From the Flory-Huggins (F-H) adsorption isotherm results for BY28-C, BY28-CC, MV-C, and MV-CC, (Tables 1 and 2), logKF-Hwas positive as a result of adsorption of both dyes on black cumin and char, and the adsorption reaction energy was calculated from Eq. 14 as ΔG kj/mol < 0. The adsorption of both dyes on black cumin and black cumin-char was spontaneous.
The conformity of BY28 adsorption to the F-H isotherm (R2) of adsorption on both adsorbent at three different temperatures is between 83% and 86% for black cumin, while it is between 93% and 98% for black cumin-char. The suitability of adsorption of MV on both adsorbent at three different temperatures to the F-H isotherm (R2) is between 82% and 96.5% for black cumin, while it is between 80% and 98.5% for black cumin-char.
Using the thermodynamic values according to the data obtained from the adsorption experiments with BY28 and MV dyes; Kads = qe/ce(L/g) using the standard thermodynamic equilibrium constant with the following equation;
. Enthalpy (ΔHo kJ/mol) was calculated from the slope of the graph plotted against lnKads versus 1/T, and entropy (ΔSo J/molK) was calculated from the cutoff point. And the Gibbs free energy (ΔGo kJ/mol) value was also calculated from the equation. Looking at the graph 10 and Table 3 for BY28-C and BY28-CC, the adsorption energy (ΔHo) indicates that it was an exothermic reaction. The entropy value was negative. This shows that the dye was adsorbed on black cumin and black cumin char, that is, it changed from an irregular state to a regular state.
Gibbs free energy values were negative. This means that the adsorption was spontaneous. For MV-C and MV-CC, the adsorption energy (ΔHo) indicates that it was an exothermic reaction. The entropy value of MV-C was negative between 25 and 150 ppm, but this value was calculated to be positive at 200 ppm. The entropy value of MV-CC was negative within the range 25 to 100 ppm, but this value was calculated to be positive at 150 to 200 ppm. Here, the negative value of ΔSo indicates that the dye changed from an irregular state to a regular state on black cumin and black cumin char. Gibbs free energy values were negative. This indicates that the adsorption is spontaneous. The ΔG values obtained from the thermodynamic calculations and the ΔG values obtained from the Flory-Huggins (F-H) adsorption isotherm were compatible with each other.
Adsorption Kinetics
The kinetic studies of BY28 and MV dyestuffs on both adsorbents at five different concentrations and three different temperatures were fitted to pseudo-first-order and pseudo-second-order rate equations. Rate graphs were drawn for 25 °C, where both dyestuffs were adsorbed the most on the adsorbents, and rate constants and qe values are shown in the Tables 5 and 6. When the experimentally found qe value and the theoretically calculated qe values in Table 5 and 6 were compared, the best value fit the pseudo-second-order velocity equation. In addition, the degree of fit R2 was close to 100% with the pseudo-second order velocity equation.
It was reported that good fits to the pseudo-second-order rate equations tend to indicate that the rate of adsorption is governed by diffusion of dye molecules within a network of very small pores (Hubbe et al. 2019). In conclusion, at 25-50-75-100-150 ppm concentrations, it was observed that BY28 satisfies the adsorption kinetic pseudo-second-order rate equation 100% for temperatures of 10-25-35 °C on black cumin and black cumin-char. In addition to these results; at 25-50-100-150-200 ppm concentrations, it was observed that the adsorption kinetics of MV on black cumin and black cumin-char at 10-25-35 °C complied with the pseudo-second-order velocity equation 100%.
Enlarge this image.Fig. 12. Graphs of ln (qe–qt) vs. t(min) of BY28-C (a), BY28-CC (c); Graphs of t/qe vs. t (min) of BY28-C (b), BY28-CC (d)
Enlarge this image.Fig. 13. Graphs of ln (qe–qt) vs. t(min) of MV-C (a), MV-CC (c); Graphs of t/qe vs. t (min) of MV-C (b), MV-CC (d)
CONCLUSIONS
1. The Freundlich adsorption capacity (KF; L/mg) constant was higher for biochar produced from cumin seed (CC), was shown in Tables 1 and 2. These findings are based on adsorption experiments carried out with basic yellow 28 (BY28) and methyl violet (MV).
2. Adsorption potential (energy) values B obtained from the Temkin isotherm, as well as heat of adsorption values E obtained from Dubinin-Radushkevich (D-R) isotherm, for both dyes were smaller than 8 kJ/mol for three different temperatures on the adsorbents. These results imply that the adsorption took place by physical adsorption.
3. When the heat of adsorption ΔHo calculated from the Temkin isotherm was compared with the heat of adsorption calculated from the thermodynamic calculation, ΔH, it was exothermic for all adsorption systems. The results of both values supported each other.
4. When the Gibbs energy ΔGo calculated from the F-H isotherm was compared with the standard Gibbs energy ΔG found from the thermodynamic calculation, the adsorption reaction was spontaneous for BY28-C, BY28-CC, MV-C, and MV-CC. The results of both values supported each other.
5. For all three temperatures, when the sorbent dose was 2 g/L (V=10 mL, m=0.02 g), its adsorption on black cumin and black cumin-char adsorbents reached equilibrium within 30 min for BY28 dyestuff, while it reached equilibrium in 60 min for MV dyestuff. As a result of adsorption, the pH value of BY28 was determined as 7 and that of MV as 8. Langmuir qmax of BY28 was found as 625 mg/g on black seed-char and 212 mg/g on black seed for 25 °C. Langmuir qmaxof MV was 909 mg/g on black seed-char and 164 mg/g on black seed for 25 °C. Thus, both black cumin and black cumin-char are expected to be very effective in removing BY28 and MV from wastewater due to its short, required retention time and high amount of adsorption on the adsorbent.
ACKNOWLEDGMENTS
The authors thank emeritus Prof. Dr. Tuba Şişmanoğlu for her experience and knowledge.
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